A pod propulsion device using an electric motor for driving the propeller shaft generates a substantial amount of heat. Since the stator of the motor normally is in contact with the housing of the motor casing a large portion of the heat is rejected through the housing to the surrounding sea water. However, the upper side of the motor casing housing does not face the water but is exposed to the interior of the strut. In this area the direct cooling to the sea water thus will not occur. Due to the low efficiency of cooling in this area a hot spot is developed. One way to coop with this is to use a motor that is larger than actually needed in order to avoid overheating within the hot spot. This is of course not a cost effective solution.
Attempts have also been made to provide specific cooling arrangements in this area. One example is disclosed in U.S. Pat. No. 6,485,339 relating to a pod with an electric motor driving a propulsion shaft. The pod is provided with means for increasing the transfer of the heat generated by the motor. For this reason there are cooling ducts arranged at various locations in the motor casing and the strut. These ducts include ducts arranged at the upper side of the motor stator, facing the strut, i.e. the region where the pod body is not in direct contact with the surrounding water. By cooling medium in these ducts such as air or water heat is rejected away from this region.
U.S. Pat. No. 7,448,929 discloses another example of providing a cooling medium such as water to flow outside the wall of the motor casing in the region where it faces the strut.
U.S. Pat. No. 7,198,528 discloses a motor casing where the motor casing is designed such that the surrounding sea water reaches all around the housing.
Further examples of cooling systems for pod are disclosed in U.S. Pat. No. 6,312,298, U.S. Pat. No. 5,403,216, U.S. Pat. No. 6,676,463, U.S. Pat. No. 6,685,516, U.S. Pat. No. 7,371,134, EP 1010614 and EP 2098447.
The provision of special cooling ducts for cooling the hot spot is circumstantial and space consuming, in particular since cooling ducts also are required to cool other parts of the electric motor.